Deuterated sevoflurane as an inhalational anesthetic

ABSTRACT

Use of D2-sevoflurane is disclosed as an inhalational anesthetic. Deuterated sevoflurane possess all of the desirable qualities of sevoflurane as an anesthetic and is metabolized more slowly thereby reducing potentially toxic inorganic fluoride release. More particularly, the compound fluorodideutero methyl 1,1,1,3,3,3-hexafluoropropyl ether is disclosed as well as a method for synthesis.

This is a divisional of copending application Ser. No. 08/010,264 filedon Jan. 28, 1993 now abandoned.

BACKGROUND OF THE INVENTION

Halogenated isopropyl derivatives of ether have demonstrated promise foruse in the medical field due to their anesthesia inducing properties. Ofthese, the most successful to date has with fluorinated isopropyl etherssuch as sevoflurane (fluoromethyl 1,1,1,3,3,3-hexafluro-2-propyl ether).Sevoflurane has demonstrated rapid induction and recovery fromanesthesia when administered by inhalation, making it attractive for useas an anesthetic. Further, sevoflurane is a volatile liquid,nonflammable in air at ambient temperatures and has a lower flammabilitylimit in oxygen of about 11.8 volume-percent, making it safe to use aswell. U.S. Pat. No. 3,683,092 to Regan et al. discloses use ofsevoflurane as an anesthetic.

While exhibiting many beneficial anesthetic properties, use ofsevoflurane as a general inhalational anesthetic has been hampered byits potential nephro-toxicity when metabolized at sufficiently highlevels.

Attempts to find other halogenated isopropyl derivatives with beneficialanesthetic properties have led scientists to substitute sevoflurane withother similar moieties. These attempts have not been successful in thatseveral related compounds either do not possess any anestheticproperties, produce only small anesthetic properties, or are toxic. Forexample, U.S. Pat. No. 3,683,092 discloses that the compound CH₃OCF(CF₃)₂ was found to be non-anesthetic up to 8% by volume in oxygenmeaning that it would burn at its anesthetic concentration since itslower flammability limit is about 7-8%. Another isomer,trifluoromethyl-2,2,3,3-tetrafluoropropyl ether of Aldrich and Shepard,Jorg., Volume 29, pages 11-15 (1964) has been shown to cause violentconvulsions and death in mice at concentrations as low as 0.5%. Yetanother isomer, CHF₂ OCH₂ CF₂ CF₃ is non-anesthetic up to its lethalconcentration and produces convulsions in mice. Still anothercomparison, it has been found that the isomeric (CHF₂)₂ CF--O--CHF₂ is aweak anesthetic in which deep anesthesia is not obtained and abnormalelectro-encephalographic and convulsant activity is observed. Thus itcan be seen that there has been little success to dater and a needexists for an anesthetic for use in animals which will possess theadvantageous characteristics of sevoflurane while minimizing theconcomitant fluoride ion release.

It is an object of the present invention to provide a compound with thebeneficial properties of sevoflurane for use as an inhalationalanesthetic which will reduce metabolic inorganic fluoride release.

Yet another object of the present invention is to provide a method forinducing anesthesia in patients involving inhalation of deuteratedsevoflurane.

It is yet another object of the present invention to provide a method ofinducing anesthesia which upon inhalation will produce anesthesia in apatient while being slowly metabolized.

A further object of the present invention is to provide a method ofsynthesis of fluoro-dideutero-methyl 1,1,1,3,3,3-hexafluoro-2-propylether.

Further objects of the invention will be demonstrated from the detaileddescription of the invention which follows.

SUMMARY OF THE INVENTION

This invention relates to a method of synthesis of deuteratedsevoflurane (D₂ sevoflurane) and use of the same for anesthetizinganimals. D₂ sevoflurane is metabolized and subsequently defluorinated ata much slower rate thereby reducing fluoride ion release, whilemaintaining all of the anesthetic properties of sevoflurane.

A method of inducing anesthesia in animals is disclosed in which D2sevoflurane is administered by inhalation to the animals. Further,fluoro-dideutero-methyl 1,1,1,3,3,3-hexafluoro-2-propyl ether issynthesized by reacting dimethyl-D₂ -sulfate with1,1,1,3,3,3-hexafluoro-isopropanol, which is then reacted with BrF₃.Excess BrF₃ is then destroyed leaving fluoromethyl1,1,1,3,3,3-hexafluoro-2-propyl ether.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph depicting concentration dependent defluorination ofD2-sevoflurane, sevoflurane, and enflurane in hepatic microsomes fromIsoniazid treated rats.

FIG. 2 is a graph depicting concentration dependent defluorination ofD2-sevoflurane, sevoflurane, and enflurane in hepatic microsomes fromphenobarbital treated rats.

FIG. 3 is a bar graph depicting Plasma fluoride levels in ratsanesthetized for 30 minutes with D2-sevoflurane, sevoflurane, andenflurane.

DETAILED DESCRIPTION OF THE INVENTION

Sevoflurane, or fluoromethyl 1,1,1,3,3,3-hexafluoro-2-propyl ether hasthe following formula: ##STR1##

Sevoflurane, upon administration into the body, is metabolized in theliver by cytochrome P450, liberating fluoride and hexafluoroisopropanol.This inorganic fluoride at sufficiently high levels will produce renaldysfunction including polyuria.

In vitro tests have demonstrated that deuterated sevoflurane,particularly with deuterium substitutions at the monofluoro substitutedmethyl group may be used in animals and as an inhalational anesthetic.One such deuterium substituted derivative found to be especially usefulis fluoro-dideuteromethyl-1,1,1,3,3,3-hexafluoro-2-propyl ether of whichthe following is a formula: ##STR2##

The compound retains all of the beneficial anesthetic qualities ofsevoflurane as discussed earlier while at the same time decreasing theexposure to fluoride. This result is surprising due to the fact thatisomers of halogenated isopropyl ethers are largely unpredictable withrespect to their anesthetic qualities. Further it has been demonstratedthat deuterium substitution of ethers used as anesthetic compounds areequally unpredictable in their altered kinetics of metabolism. Anothercompound which may be useful in the present invention isfluoro-dideuteromethyl-1,1,1,3,3,3-hexafluoro-2-deutero-2-propyl ether(D₃ -sevoflurane).

U.S. Pat. No. 4,154,971 to Larsen et al. discloses monodeuteratedanalogs of 1,1 difluoro 2,2-dihaloethyl difluoromethyl ethers.Accordingly it was discovered that1,1,2-trifluoro-2-chloro-2-deuteroethyl difluoromethyl ether;1,1-difluoro-2-deutero-2,2-dichloroethyl difluoromethyl ether; and1,1,2-1-trifluoro-2-bromo-2-deuteroethyl difluoromethyl ether allexhibited the characteristic of slower metabolism and thus slowerdefluorination. However, 1,1-difluoro-2,2-dichloro-2-deuteroethyl methylether exhibited properties of being more readily metabolized intoinorganic fluoride than the undeuterated compound. This unpredictabilityof deuteration on fluoride ion release has similarly been encountered inother patented systems.

U.S. Pat. No. 4,153,636 similarly discloses deuterated analogs ofmethoxyflurane wherein 2,2-dichloro-1,1-difluoro-1-methoxy-d₃ -ethane-dand 2,2-dichloro-1,1-difluoro-1-methoxy-d₃ -ethane were found to havedecreased metabolism and again 1,1 difluoro-2-2-dichloro-2-deuteroethylmethyl ether was found to increase organic flouride release.

The unpredictability of deuteration of anesthetics can also bedemonstrated by the deuteration of halothane. Deuteration of halothaneinhibits its metabolism to trifluoroacetic acid, but not its metabolismto release fluoride (Sipes IG, Gandolfi AJ, Pohl LR, Krishna G, Brown JrBR: Comparison of the biotransformation and hepatotoxicity of halothaneand deuterated halothane. J. Pharmacol. Exp. Ther. 14:716-720, 1980).

Thus it can be seen that the placement of deuterium atoms in themolecule is critical and highly species specific. D₂ sevoflurane withdeuterium atoms at the monofluoro methyl group produces an unexpectedunpredictable result of decreased rate of metabolism and decreasedfluoride ion release while maintaining all the beneficial anestheticproperties of the compound.

According to the present invention, substitution of the hydrogens at themethyl group of sevoflurane with deuterium(D), a heavy isotope ofhydrogen, alters the kinetics of metabolism of the compound. Thecompound retains its anesthetic qualities while being metabolized muchmore slowly thereby reducing production of inorganic fluoride.Substitution of the hydrogens at the methyl group of sevofluraneeliminates the concentration-dependent peak of fluoride release whichoccurs upon sevoflurane and enflurane metabolism in microsomes fromisoniazid treated rats. Liver mocrosomes from isoniazid treated ratscontain the same cytochrome P450 isozyme, P450IIE1, which is present inhumans, and is inducible by ethanol and other compounds in humans.

D2-sevoflurane may be synthesized by modification of a method forsynthesis of sevoflurane described in U.S. Pat. No. 3,683,092, thedisclosure of which is incorporated herein by reference. The method issimilar except, Dimethyl-D6-sulfate, instead of dimethyl sulfate isreacted with 1,1,1,3,3,3-hexafluroisopropanol to formtrideuteromethyl-1,1,1,3,3,3-hexafluoro-2-propyl ether. The resultingtrideuteromethyl-1,1,1,3,3,3-hexafluoro-2-propyl ether is thensubsequently monofluorinated by reaction with bromine trifluoride.

D3-sevoflurane is synthesized by reacting D2-sevoflurane with NaOD in D₂O. The D2 sevoflurane is deuterated in the 2 propyl position to formfluoro-dideuteromethyl-1,1,1,3,3,3-hexafluoro-2-deutero-2-propyl ether.All reactions are run with equimolar quantities of reactants preferred,although excesses of one or more of the variants may be used. Nocritical limits as to temperature or pressure exist, traditionallyambient conditions will be used.

The end product, D2-sevoflurane or D3-sevoflurane may then beadministered by the inhalation route to warm blooded, air breathinganimals, in an effective anesthetizing amount. Generally the compound isadministered in an amount of from about 1 percent to about 5 percent byvolume in admixture with from 99 percent to about 95 percent by volumeof oxygen or a gaseous mixture containing oxygen and/or otheranesthetics in sufficient amount to support respiration.

EXAMPLE 1

To prepare trideuteromethyl-1,1,1,3,3,3-hexafluoro-2-propyl ether,hexafluoroisoproponal (53.3 g) was added to 127 ml of 108 aqueous sodiumhydroxide in a Pyrex flask. Dimethyl-D6-sulfate (40 g) was addedproportion wise during a thirty-minute period at 5 ° C. while stirring.The reaction mixture was stirred for two hours at room temperature.Distillation of reaction mixture yielded 45 g oftrideuteromethyl-1,1,1,3,3,3-hexafluoro-2-propyl ether.

D2-sevoflurane was obtained by placing 8 ml of driedtrideuteromethyl-1,1,1,3,3,3-hexafluoro-2-propyl ether in a Pyrex flask.3 ml of BrF₃ were slowly added over a two hour period while stirring. Anexothermic reaction occurred, monofluorinating the ether compound.Following the reaction, water was cautiously added to destroy excessBrF₃ in the reaction mixture. The reaction mixture was successivelywashed with dilute sodium sulfate and water. Finally the washed mixturewas dried over anhydrous sodium sulfate and yielded 3.1 mlD2-sevoflurane.

EXAMPLE 2

The formation of D2-sevoflurane and determination of its purity wereevaluated by two methods of gas chromatography, and by GC-massspectrometry using the electron impact (EI) and chemical ionization (CI)modes.

The synthesized product exhibited a retention time identical to that ofsevoflurane on gas chromatography. D2-sevoflurane chromatography on acarbowax column and using flame ionization detection, showed that theD2-sevoflurane was 99.86% pure. The contaminant at 7.0 minutes andconstituting 0.0328 of the sample was identified ashexafluoroisopropanol. Chromatography of the synthesized D2-sevofluranesample on 10% CO-880 15% LB-550X indicates a purity of 99.9%. Thiscolumn resolves methyl hexafluoroisopropyl ether or trideuteromethylhexafluoropropyl ether (retention time of 2.2 minutes) from sevofluraneor D2-sevoflurane (3.3 minutes) and showed that the sample contained notrideuteromethyl-1,1,1,3,3,3-hexafluoro-2-propyl ether (or methylhexafluoroisopropyl ether).

EXAMPLE 3

Mass spectral analysis of the synthesized D2-sevoflurane was performedon a Nermag R10-10C mass spectrometer in the electron impact andchemical ionization modes. The mass spectrometer was equipped with a DBWax 30 m×0.2 mm×0.5 μm capillary column for sample introduction.

The electron impact mass spectra of sevoflurane and D2-sevoflurane wereobserved. The parent ion of either compound was not observed; however,the M-F and M-CF3 fragments occurred. Sevoflurane analysis yielded a M-Ffragment with m/z of 181, whereas D2-sevoflurane produced a fragment ofm/z 183--two atomic mass units greater. Also, sevoflurane generated am/z fragment of 131 (M-CF3), whereas D2-sevoflurane showed thecorresponding fragment at m/z 133--also two atomic mass units greater.The greater mass of 2 for these fragments confirms that the deuteratedcompound is fluoro-dideutero-methyl-1,1,1,3,3,3-hexafluoro-2-propylether, and the spectra showed that in the D2-sevoflurane sample nosevoflurane was detectable.

Mass spectra of sevoflurane and D2-sevoflurane in the chemicalionization mode showed the parent ion m/z (M+1) of 201 for sevofluraneand 203 for D2-sevoflurane. The parent ion of D2-sevoflurane was 2atomic mass units greater than that of sevoflurane again confirmingD2-sevoflurane.

EXAMPLE 4 -- METABOLISM OF D2-SEVOFLURANE

The metabolism of D2-sevoflurane is expected to liberate one fluorideion for each molecule metabolized by cytochrome P450 since themetabolism of sevoflurane liberates fluoride and hexafluoroisopropanol.To determine the metabolism of D2-sevoflurane relative to sevofluraneand enflurane, these anesthetics were incubated with hepatic microsomesfrom untreated male Sprague-Dawley rats (200-230 g), or rats treatedwith isoniazid (80 mg/kg, i.p. for 5 days), or phenobarbital (0.28 inthe drinking water for 4 days). Isoniazid induces the cytochrome P450isozyme P450 2E1 which is thought to metabolize the volatileanesthetics, and phenobarbital induces several forms also shown to playa role in anesthetic metabolism in the rat.

Each incubation vial (6 ml plastic vial) contained 3 ml of 5 mg/mlmicrosomal protein in a 0.1M sodium phosphate buffer, pH 7.4. An NADPHgenerating system was added to cytochrome P450 activity, and the NADPHgenerating system was omitted from control incubations. Anesthetic wasadded in the quantities indicated and microsomes were incubated for 15minutes at 37 ° C. Reactions were stopped by placing the vials on ice.Fluoride was assayed in the microsomal mixtures using fluorideion-specific electrodes (Fisher Scientific) and a 720A Orion pH/ISEmeter. Following incubation microsomes were mixed with an equal volumeof TISABII buffer for fluoride analysis. Fluoride in each sample wasdetermined from standard curves constructed using fluoride standards(10⁻⁷ to 10⁻³ M NaF) prepared from a commercially available standardsolution (10⁻¹ M NaF).

Comparison of the defluorination of D2-sevoflurane and sevoflurane inmicrosomes incubated with an excess of either anesthetic (1 μlanesthetic per incubation) shows that D2-sevoflurane is defluorinatedmuch slower than sevoflurane in all microsomal preparations (table 1).

                  TABLE 1                                                         ______________________________________                                        COMPARATIVE DEFLUORINATION OF SEVOFLURANE                                     AND D.sub.2 -SEVOFLURANE BY RAT LIVER MICROSOMES*                             nmol F.sup.- /mg protein/30 min ± S.E.                                     Animal Treatment                                                                            Sevoflurane                                                                              D.sub.2 -Sevoflurane                                 ______________________________________                                        None          1.94 ± 0.31                                                                            0.62 ± 0.60 (68)*                                Isoniazid     7.46 ± 0.83                                                                           1.55 ± 0.39 (79)                                  Phenobarbital 1.18 ± 0.06                                                                           0.18 ± 0.03 (84)                                  ______________________________________                                         *Numbers in parentheses represent percent decline from sevoflurane values     following correction for background (0.43). Values represent the mean and     standard errors of triplicate determinations.                            

The degrees of inhibited metabolism are 68, 79 and 84% in microsomesfrom untreated, isoniazid and phenobarbital treated rats, respectively.The concentration-dependent defluorination of D2-sevoflurane,sevoflurane and enflurane, in microsomes from phenobarbital andisoniazid treated rats show that over a wide range of anestheticconcentrations D2-sevoflurane is defluorinated substantially slower thansevoflurane (70-86% less) or enflurane (FIGS. 1 and 2). In microsomesfrom isoniazid treated rats in which the metabolism of all anestheticsis the greatest due to the induction of P450 IIE1, there was ananesthetic concentration-dependent inhibition of metabolism bysevoflurane and enflurane, but not D2-sevoflurane (FIG. 1). These datasuggests a substrate inhibition phenomenon. In microsomes from ratstreated with phenobarbital this did not occur (FIG. 2).

EXAMPLE 5 -- IN VIVO METABOLISM OF D2-SEVOFLURANE

Untreated rats or rats treated with isoniazid or phenobarbital wereexposed to D2-sevoflurane, sevoflurane, or enflurane to determine therelative rates of fluoride production in vivo.

The animals were exposed in a 3.8 L plastic exposure chamber with anatmosphere of 100% oxygen. Male Sprague-Dawley rats (200-220 g, 4 pergroup) were placed in the chamber and the chamber was flushed withoxygen and sealed. Anesthetic was introduced into the chamber liquidform via an injection port. Quantities were introduced to give initialconcentrations of 3% anesthetic (enflurane, 464 μl; sevoflurane andD2-sevoflurane, 524 μl). The rats became anesthetized within 4-6 minutesafter introduction of each anesthetic. Oxygen and carbon dioxide weremonitored periodically during the exposure period with an Ohmeda 6000multi-gas monitor.

Following a 30 minute exposure period, the chamber was flushed with 100%oxygen for 5 minutes and the animals quickly awoke. The rats wereimmediately removed and injected i.p. with 80 mg/kg secobarbital. Whileanesthetized 3 to 4 ml of blood were withdrawn by cardiac puncturewithin 15 minutes of termination of anesthetic exposure (within 10minutes of removal from the chamber). Plasma was prepared and fluorideanalyzed as described above.

Exposure to D2-sevoflurane resulted in lower plasma fluoride thanexposure to either enflurane or sevoflurane (FIG. 3). As compared to Theliberation of fluoride from sevoflurane, D2-sevoflurane liberated 61%less in isoniazid treated rats, 66% less in phenobarbital treatedanimals, and 34 less in untreated rats. D2-sevoflurane also liberatedless fluoride than enflurane in vivo. In untreated, and isoniazid andphenobarbital treated rats, the plasma from D2-sevoflurane exposed ratscontained 40, 80, and 45%, respectively, less fluoride than enfluraneanesthetized animals.

What is claimed is:
 1. An inhalation pharmaceutical composition for inducing anesthesia in animals said composition comprising:an anesthesia inducing effective amount of fluorodideutero 1,1,1,3,3,3-hexafluoro-2-propyl ether and a pharmaceutically acceptable anesthetic carrier.
 2. The composition of claim 1 wherein said carrier is oxygen.
 3. The composition of claim 1 where said composition comprises in gaseous mixture from about 1% to 5% of volume of fluorodideutero methyl 1,1,1,3,3,3-hexafluoro-2-propyl ether and 99% to 95% by volume oxygen or oxygen in mixture with other components at a sufficient level to support respiration.
 4. The pharmaceutical composition of claim 1 wherein said composition is a gaseous mixture comprising from about 1% to about 5% of volume of said fluorodideutero methyl 1,1,1,3,3,3-hexafluoro-2-propyl ether. 